A single, unexplained particle interaction recorded nearly a mile underground in South Dakota has intrigued scientists. It is not yet a confirmed detection of dark matter, but it is precisely the kind of event that makes researchers stop and think.
An anomalous event in the Black Hills
Researchers from the LUX-ZEPLIN (LZ) dark matter experiment have reported the detection of a single particle interaction that has generated significant interest. While it is not yet a confirmed sighting of dark matter, the event is one of those anomalies that could give scientists pause. The findings have been published on the arXiv platform and submitted to the journal Physical Review Letters.
Dark matter is a hypothetical form of matter believed to constitute 85 per cent of all matter in the universe and nearly a third of its total mass and energy. We do not yet know what it is, but its existence is inferred from its gravitational influence; crucially, it does not interact with light, hence the name.
A subterranean trap for WIMPs
To attempt to capture these elusive particles, the LZ experiment uses a tank filled with ultra-pure liquid xenon, situated approximately 1,500 metres (nearly a mile) below the surface at the Sanford Underground Research Facility (SURF) in South Dakota. The location is critical: at such a depth, the detector is shielded from the cosmic rays that constantly bombard the earth’s surface and would otherwise create interfering noise.
One leading theory posits that dark matter consists of Weakly Interacting Massive Particles, or WIMPs. These particles are heavier than the standard particles that make up our bodies but rarely interact with them. On occasion, a WIMP may collide with ordinary matter, causing a nuclear recoil. This, in turn, produces a flash of light that can be measured by the detector.
However, the team must meticulously filter out background events. Naturally occurring radioactive phenomena in the surrounding environment or within the detector itself can be mistaken for a genuine signal.
A needle in a haystack
The detected particle, designated LZ230616, points to a particle with a mass over 200 times that of a proton, and the event unfolded in a manner that does not match any known background process.
“This event, which we have labelled LZ230616, stands out from the others,” said lead author Sam Eriksen from the University of Bristol. “We observed it in a region of parameter space where background activity is incredibly low, and the event does not behave as we would expect from background noise. In simple terms, this means we may have detected something new. One explanation is an extremely rare background event, but another, more exciting, possibility is that it could be the first sign of dark matter.”
The event’s statistical significance is 2.6 sigma, which is well below the five-sigma gold standard required for a discovery in particle physics. This translates to a roughly 1 in 200 chance that the event is simply a background fluctuation.
One might think that a 99.5 per cent confidence level for a new particle is quite high, but would you get into a car that had a one-in-200 chance of exploding on each journey.
The five-sigma requirement is necessary to achieve the requisite level of certainty. In other experiments, tantalising new particles have vanished into thin air as more data was collected. The team fully intends to gather considerably more information.
The long road to confirmation
“This is one of the events recorded in the data collected over 370 days. To say anything more about this event or similar, as-yet-unseen events, we need to analyse more data that LZ continues to collect,” said Eriksen.”Even with all the data LZ is expected to gather until 2028, dark matter interactions are so rare that we would still only expect to see a handful of events. We are presenting these results to the scientific community to get their input on what this event might have been.”
The dark matter hypothesis has been a leading theory for decades, but despite all efforts, confirming its existence and nature has remained elusive.
“We are gradually building larger and more complex detectors, and LZ is the most sensitive to date. In our latest search, we recorded an event that does not resemble any known background phenomenon. It could still be an extremely rare background event that no one has seen before, but it could also be the first hint of observing dark matter,” said Eriksen.
A new background event would be very exciting, but if this is the first compelling evidence for WIMPs, it would be revolutionary.
